Modern electronic scales produce a weight reading by converting the force of a weight on a weighing pan, weighing platform or other load receiver into an electronic signal. The form of this signal as a function of the weight on the weighing platform often shows deviations from linearity. In such cases the weight displayed by the scale does not agree with the true weight over the entire range of measurement.
In order to check the linearity of electronic scales and correct for nonlinearities, it has been known to place reference weights of precisely known mass on the weighing platform, to compare the weights displayed with the known actual weights and to use the deviations between the known and measured weight values thus found to correct subsequent measurements using the electronics of the scale. However, reference weights of mass known to the precision required for conventional linearization procedures tend to be expensive. Such weights are subject to loss and theft and, if not treated with care, can become altered in mass and uncalibrated. A need for reference weights of precisely known mass for linearizing an electronic scale tends to defeat a principal advantage of electronic scales over conventional dual-pan balances, namely that electronic scales ordinarily do not require a set of precision calibrated weights to make weight measurements, whereas dual-pan balances require such a set of weights.
One example of a conventional nonlinearity correction procedure for electronic scales is disclosed in U.S. Pat. No. 3,976,150, which proposes to place four precisely known weights on an electronic scale one after the other, to store the measured values and then make use of the stored values for the calculation of linearization coefficients.
A similar correction procedure is followed in European patent application No. 0044707. A measured value of a precisely known reference weight is stored. Subsequently the difference between the stored measured weight and the known weight is calculated. The resulting value is then taken into account by a calculator in the scale for correcting the measurement of weights subsequently placed on the scale.
In German published patent application Ser. No. DE-A1-3 144 103 an electronic scale is disclosed which uses a reference weight of 90-100 percent of the maximum load for a calibration procedure. Initially, the magnitude of the reference weight is measured on the scale and stored. In order to carry out a secondary calibration of the scale, the reference weight is again placed on the scale, its weight is measured and the result is compared with the previously stored value. The calibration may then be carried out by certain evaluation electronics of the scale using the difference between the two measured values of the reference weight. Although this calibration apparatus makes possible a precise determination of a single reference point, verification and correction of the linearity of the scale over the entire range of measurement is not accomplished.